C is the official answer
I would like C better if it said "high air fares" instead of "low air fares". The stem tells us that in spite of rising fares demand keeps going up. So the proper inference should be that something else besides the price is making demand to rise.
The argument is based on the premise of the relationship between low air fares and high passenger miles flown. The increase in the air fares should have resulted in a decrease in passenger miles flown. but it didnt. So isnt it reasonable that our PREMISE was not correct.